Sulfone compounds are used as extraction solvents or various reaction solvents, and sulfone compounds with a high dielectric constant are also used as solvents for an electrochemical device as aprotic polar solvents. Specifically, proposed use of sulfone compounds as a solvent are seen in devices such as an electric double layer capacitor in which sulfone compounds such as sulfolane and sulfolane derivatives (e.g. 3-methyl sulfolane) are used as an electrolyte (Patent Document 1); and an electric double layer capacitor in which a mixed solvent of propylene carbonate and at least one of sulfolane and a sulfolane derivative such as 3-methyl sulfolane is used as an electrolyte (Patent Document 2)
Aprotic polar solvents used as a solvent for an electrochemical device and the like generally desirably have a low melting point and excellent thermal stability. Depending on types of an electrochemical device, the existence of water within the system can be a problem. In such a case, a solvent with low solubility of water therein is preferably used.
However, sulfone compounds disclosed in Patent Documents 1 and 2 have a relatively high melting point, and therefore have problems such as a decrease in function in a low-temperature environment. Propylene carbonate used together with these sulfone compounds has problems such as inferior thermal stability and relatively high solubility of water in the propylene carbonate.
Patent Document 1: Japanese Kokai Publication Sho-62-237715 (JP-A Sho-62-237715)
Patent Document 2: Japanese Kokai Publication Sho-63-12122 (JP-A Sho-63-12122)